April 2, 2007

The latest news continues to be full of stories about global warming, and one of the constant pillars of the apocalypse is that agricultural yields will substantially decline due to higher temperatures, increased drought, spread of diseases, invasion of weeds, destruction of soil nutrients, and … you name it! We did a quick search of the internet for “Global Warming and Agriculture” and found more than 5,000,000 websites, and as we began sampling the sites, we encountered an overwhelming amount of bad news. Occasionally, we would find sarcastic comments about “growing barley in Iceland,” but overall, we found gloomy news about our agricultural future. Of course, within the first two sites visited, we learned that “the region likely to be worst affected is Africa, both because its geography makes it particularly vulnerable, and because seventy percent of the population rely on rain-fed agriculture for their livelihoods.”

We at World Climate Report have long questioned such a pessimistic view of our future. Literally thousands of experiments have been conducted showing that agricultural plants benefit enormously in environments of higher concentrations of atmospheric carbon dioxide – they increase their rate of photosynthesis, increase water use efficiency, and increase yields. Furthermore, hundreds of experiments have shown that many agricultural plants benefit from higher temperatures, particularly higher temperatures at night. Believe it or not, most agricultural plants benefit from less frost! With all the gloom and doom about increased drought in the future, we note that all climate models predict increased precipitation on a global scale with little ability to predict changes in precipitation at local or even regional scales. Finally, can you name any important agricultural crop that has seen a reduction in yield per unit area over the past century? You cannot, because years of agricultural research have improved both the plants and the farming practices. Our guess is that the research in the future will produce even greater increases in yields, despite any changes that occur to the climate.

OK – our juices started flowing again on this topic given a recent article in the journal Climatic Change entitled “Historical effects of temperature and precipitation on California crop yields.” As the title suggests, the article is about how crop yields in the real world (well, California) have been impacted by observed variations in temperature and rainfall; the article is not some numerical modeling study involving plants existing only within the electrons of computer chips.

The Lobell et al. team is made up of members from Lawrence Livermore National Laboratory, the Carnegie Institution, and Stanford University. They begin their article noting that “Agriculture in California represents an economically and culturally important activity that contributes substantially to national and international production of many commodities. Statewide agricultural income from sales in 2003 was $27.8 billion, or 13% of the U.S. total. As the nation’s leading producer of 74 different crops, California supplies more than half of all domestic fruit and vegetables.” Furthermore, they state “Despite advances in technology and the widespread prevalence of irrigation in the state, agricultural production remains highly dependent on weather, which can affect both the quantity and quality of harvested crops.”

We could find articles every week about how corn, wheat, soybeans, and many other such crops grow better with increasing CO2 levels. In fact, the literature is so littered with these articles that it tends to get boring, but in this California study the 12 crops include wine grapes, lettuce, almonds, strawberries, table grapes, hay, oranges, cotton, tomatoes for processing, walnuts, avocadoes, and pistachios. Ah, something a bit more interesting!

The research team is fully aware that yields over time are impacted by changes in technology (and increases in atmospheric carbon dioxide concentrations), and the effect was eliminated statistically by expressing yields as deviations from a linear upward trend. Also, “many fruit and nut crops exhibit alternate bearing, with years of high reproductive growth (high yields) alternating with years of high vegetative growth (low yields).” This effect was controlled for using an autoregressive statistical model which for most crops is insignificant.

The trio collected daily minimum temperature, maximum temperature, and precipitation data for 382 stations in California, and they collected the yield data for the 12 crops from 1980 to 2003. As seen below, despite all the gloom and doom from the five million websites on global warming and agriculture, we certainly see an upward trend in yields in virtually every crop they examined!

Figure 1. Average statewide yields in tons per acre (black line) and total statewide area in 1000 acres (gray line)
for individual crops from 1980–2003 (from Lobell et al., 2007).

With respect to how yields varied with changes in temperature, they report the following:

• “Wine grape yields were favored by years with warm nighttime temperatures in April and higher rainfall in June. Warm April temperatures reflect decreased risk of frost damage during the vulnerable post-budbreak spring growing period, when frosts can severely depress yields by damaging rapidly developing vegetative and cluster tissues”

• “Lettuce yields appear aided by warm days in April (up to about 23°C), as well as the October prior to harvest year.”

• “Table grape yields were increased with October rains in the year preceding harvest, and with warm nighttime July temperatures.”

• “Orange yields were most correlated with ppt [precipitation] in May and tmin [minimum temperatures] in December prior to and March of the harvest year.”

• “The strongest climatic response variable for cotton was a positive effect on yield for warmer May tmax [maximum temperatures]”

• “Tomato yields increased with warm April tmax, and with June tmax up to 32°C”

Not all the results were so positive – almonds and walnuts prefer cooler nights in February, strawberries like cool temperatures in November, avocadoes were sensitive to both warm and cold depending on development of the fruit, and hay yields increased in cooler winter seasons. They conclude that “Overall, climate changes since 1980 have had a mixed effect on yields of these 12 crops, with a small net effect on the food crop sector.”

So during the time of greatest greenhouse gas buildup, during a time of “unprecedented” warming (according to the five million websites), crop yields in California increased, many crops seem to benefit from warming, others do not, and overall, the variations in climate had a “small net effect” of California agriculture.